The alkali contents of process streams and particularly coal-derived gaseous fuels and their products of combustion, are important parameters to accurately assess due to their potential for corrosion and erosion of downstream components, such as gas turbines, in hot environments. Current industrial standards for turbine fuels require less than 70 parts per billion (ppb) of alkali for 1,340 Kcal/m.sup.3 fuel gas and one ppm by weight for particles in the gas fed into the turbine. After an extensive study corrolating alkali concentration with turbine-blade wear, it has been determined that to achieve 25,000 hours of use (an acceptable lifetime) from a gas turbine, the level of alkali in the fuel stream should not exceed 24 ppb. In order to assess if fuel gases are meeting gas turbine and other component standards, analytical methods are required for the measurement of alkali concentration in high-temperature process streams.
Typically, alkali measurements in process streams are made by laboratory analysis of sodium and potassium content of particles which are collected by filters. Also, alkali measurements are made by laboratory analysis of the sodium and potassium content of condensates which are collected by impinger trains from the gas streams. Both methods require long sampling periods of between four and eight hours and generate only a time-averaged value. These methods cannot provide information important to assess short-term, transient behavior of alkali loadings in the system. It is important to assess these short-term loadings by real-time analysis due to their damage potential to downstream units and because of their role in determining process stability or diagnosing process instability. Further, assessment of short-term loadings is important in order to take corrective action if the alkali levels exceed component standards. In addition, to accurately assess the alkali content of fuel and exhaust streams at component specification levels, the alkali monitoring technique must also possess detection limits in the part per billion concentration range. A wide dynamic range is also an essential attribute so that streams containing both part per billion and part per million levels can be monitored, and to allow for fast, accurate tracking of transient alkali behavior. In order to respond to this need for real-time transient alkali data often at low part per billion levels, a number of techniques have been recently investigated. A hot wire ionization monitor has been tested on a fluidized-bed combuster. This technique determines alkali levels by measuring the current generated upon alkali impaction and subsequent ionization on a platinum wire positioned in the gas stream. However, it was shown in preliminary tests that quantitative information on alkali content of process streams was not achievable.
Another in situ alkali monitoring device is being developed which utilizes laser-induced photoionization spectroscopy to identify gas-phase alkali species. Alkali speciation is possible using this technique, although its application to routine-on-line process stream analyses is uncertain due to its expense and potential calibration problems.
A flame emission alkali monitor system composed of a burner, a light collection system, and a computer-controlled, dual-channel spectrometer has also been developed. In this device, a sample of a gas stream is introduced into the flame produced by the burner which is fueled with either an oxygen-hydrogen or nitrous oxide-acetylene mixture. The alkali containing species are vaporized, and the alkali atoms are thermally excited. The intensities of the sodium and potassium emission lines are measured simultaneously by two 0.25 meter focal length monochromators. This analyzer yields an on-line measurement every one to five minutes and has been successfully tested at various gasification and fluidized-bed combustion facilities.
Based on the flame atomic emission technique, an extractive, total (vapor and particle-bound) sodium and potassium monitor has been designed, constructed and tested. This monitor is described in U.S. Pat. No. 4,616,137 issued on Oct. 7, 1986. In that system, alkali flame emissions are focused on one branch of a bifurcated fiber optic cable and guided to a first bandpass filter adapted to the common trunk end of the cable. A portion of the light is allowed to pass through the filter to a first detector and the remaining light is reflected back through the common trunk portion of the fiber to a second bandpass filter adapted to the end of the other branch of the cable. The first filter is centered at a wavelength corresponding to the emission line with a small bandwidth. The second filter is centered at the same wavelength but has a larger bandwidth. Two light detectors are located to view the light passing through the filters. The second detector is blind to light corresponding to the emission line of interest which is detected by the first detector and the difference between the two detector outputs is used to determine the intensity of the combustion flame emission of interest.
The system described in U.S. Pat. No. 4,616,137 is relatively inflexible and cannot be easily adapted to detecting two or more alkali species simultaneously. In addition, it relies on an estimate of the background radiation that falls within the spectral region of interest thus creating a potential for uncertainty in the measurements from the system.
Accordingly, there is a need in the art for a real-time alkali monitoring system which is capable of accurately assessing alkali concentration in process streams.